1. Field of the Invention
The present invention relates generally to a fibre optic magnetic sensor or transducer and specifically to a magnetic transducer of the type which utilizes the magnetostrictive properties of metal alloys.
2. Description of the Prior Art
The possibility of sensing magnetic fields by the use of optical fibres has long been recognized and several types of fibre optic magnetic sensors have been developed. The most successful and sensitive devices utilize the magnetostrictive properties of metal alloys.
Magnetostriction is defined as the change in dimension of a ferromagnetic material when it is placed along the axis of an applied magnetic field. The magnetostriction may be positive (expansion) or negative (contraction) depending on the material and/or its composition. For example, pure nickel displays negative magnetostriction whereas iron-nickel alloys having less than 45% nickel display positive magnetostriction. In fibre optic magnetic sensors, a single mode optical fibre is bonded to or sheathed by a magnetostrictive jacket that undergoes a longitudinal strain when immersed in a magnetic field. This strain is transmitted to the optical fibre and affects the phase delay of a laser light beam propagating through the fibre. Provided the magnetic field is of sufficient strength, the phase changes can be detected using currently available technology and the strength of the field can be determined. The difficulty heretofore has been the provision of a relatively simple and inexpensive sensor of the type described which is capable of sensing very weak magnetic fields, that is fields in the order of 10.sup.-5 and 10.sup.-8 Gauss.
As suggested above, nickel and amorphous iron alloys have been found to be particularly suitable for use in fibre optic magnetic sensors. Amorphous alloys possess large magnetostriction but are available only in very thin (0.001 inch) strips and, thus, their versatility is rather limited. Bulk nickel transducers, on the other hand, can be configured in a variety of shapes with relative ease. However, they possess smaller magnetostriction which renders them somewhat insensitive for detecting weak magnetic fields of the magnitude in question. A further drawback of conventional optic fibre magnetic transducers is their susceptibility to and inability to be compensated for thermal effects, it being well known that thermal effects can easily swamp magnetic signals of the magnitude of interest.